1. Field of the Invention
The present invention relates to a technique of a crystal oscillator composed of discrete elements, and more particularly, to a technique for preventing a metal cover, which is used for a surface-mount oscillator of a voltage control type, from separating at the time of solder reflow.
2. Description of the Related Art
In recent years, a surface-mount crystal oscillator (surface-mount oscillator) of a voltage control type has been incorporated into various types of electronic appliances, for example, as a reference frequency source of a PLL circuit, since its oscillation frequency in response to a control voltage is stable owing to a crystal unit.
FIGS. 1A and 1B are schematic diagrams explaining a conventional example of a surface-mount oscillator. FIG. 1A is its disassembled view, whereas FIG. 1B is an enlarged cross-sectional view of a portion of both ends taken along a line A-A of FIG. 1A.
A surface-mount oscillator has a configuration where a crystal unit 2a, an IC 2b, and other circuit elements 2c are arranged on a mounting board 1, which is covered with a metal cover 3. The mounting board 1 is rectangular in shape when viewed from a plane, and comprises at least four external terminals not shown on an outer bottom of both sides of a width direction. Each of the external terminals extends to the outer bottom via a side electrode 5 provided in a groove 4a in a central region.
The metal cover 3 is concave in shape, and comprises clearances 6 from opening end faces at both sides in the width direction, and the opening end faces are made to touch the perimeter surface of the mounting board 1. In central portions at both ends in the longitudinal direction of the metal cover 3, protruding parts 8, each of which has a protrusion 7 provided on an inner face, are comprised. Each of the protruding parts 8 is elastically inserted in a groove 4b provided on both side faces in the longitudinal direction of the mounting board 1.
The groove 4b has a bottom wall layer on the side of the outer bottom, and a metal film 9 the material of which is the same as the side electrode 5 is provided on the entire inner face. The primary coat of the metal film 9 is, for example, Cu (copper), and its surface is Au-plated. Each of the protruding parts 8 of the metal cover 3 is bonded to the metal film 9 of the groove 4b by solder 10 from the outer surface. Note that the groove 4b is broader than the width of the protruding part 8. As a result, the solder 10 comes inside from the perimeter of the protruding part 8, and entirely bonds to the metal film 9.
Also protruding parts 11 for alignment are provided on both side faces at one end in the longitudinal direction of the metal cover 3. Each of the protruding parts 11 is inserted in a groove 4c of the mounting board 1, so that the metal cover 3 is pre-aligned. Such surface-mount oscillators are mounted on a set board not shown, for example, with solder reflow. The solder includes lead-free solder.
Additionally, the proposal recited in Japanese Published Unexamined Patent Application No. 2005-151536 is made.
However, the surface-mount oscillator having the above described configuration has a problem that the metal cover 3 is separated or displaced due to the melting of also solder coated on the grooves 4b at both sides in the longitudinal direction at the time of user's mounting with solder reflow. This problem conspicuously occurs, for example, when a user mounts surface-mount components on a set board the both surfaces of which are used for mounting.
Namely, this problem is caused, for example, by an upside-down surface-mount oscillator that is beforehand provided on one surface of a set board, which is then reversed when other surface-mount components are solder-reflowed on another surface. Namely, this problem occurs due to the low elastic force of the protrusions 7 of the protruding parts 8 in the surface-mount oscillator.